Methods of forming color printings, color duplicates or color proofs (proofs for printing) which comprises conducting development with electrophotographic developing agents to form a plurality of overlapping color toner images directly on the surface of electrophotographic light-sensitive element and transferring at once the resulting color images onto a receiving material such as printing paper are known.
The developing methods include a so-called dry type developing method and wet type developing method. Color images obtained by the wet type developing method are preferred because of little color shear and good resolution as compared with those formed with dry toners. However, it is very difficult to directly transfer wet type toner images entirely from the surface of the light-sensitive element to printing paper.
In order to solve this problem, a transfer technique in which a non-aqueous solvent is supplied between a light-sensitive element and a receiving material and then transfer is electrostatically performed is described in JP-A-2-272469 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Also, a method in which a transparent film is first laminated on the surface of a light-sensitive element, wet type toner images are formed by an electrophotographic process on the film, and then the film bearing the toner images is separated from the light-sensitive element and stuck on plain paper, thereby forming transferred images is described in JP-A-2-115865 and JP-A-2-115866. According to the method, the film to be laminated has suitably a thickness of 9 .mu.m. However, the production and handling of a film having such thickness is very troublesome and it is necessary to arrange a special system for them.
Further, in JP-B-2-43185 (the term "JP-B" as used herein means an "examined Japanese patent publication"), a method in which imagewise exposure through a transparent electrophotographic light-sensitive element and development are conducted repeatedly to form overlapping color separation images on a dielectric support releasably provided on the light-sensitive element and the dielectric support bearing the images is transferred to a receiving material is described. Since the imagewise exposure is performed from the side of substrate for the light-sensitive element according to this method, the substrate is required to be transparent. This is disadvantageous in view of a cost.
On the other hand, an electrophotographic transfer method using a so-called dry type developing method in which a releasable transfer layer is provided on the surface of a light-sensitive element, toner images are formed on the transfer layer and the toner images are transferred together with the transfer layer to printing paper is described in JP-A-1-112264, JP-A-1-281464 and JP-A-3-11347.
However, in order to employ the light-sensitive element repeatedly in the techniques hitherto known, a special operation is required at the time of transfer or difficulties in the formation of transfer layer are encountered. On the other hand, the method using a light-sensitive element having a transfer layer (or a releasable layer) previously formed thereon is disadvantageous in point of cost since the light-sensitive element used is inevitably thrown.
Moreover, in JP-A-2-264280 a method in which toner images on a light-sensitive layer are transferred onto a primary intermediate transfer medium having high smoothness and then transferred onto a final receiving material is described. Also, a method in which a special transfer medium is used in order to obtain final color images of good quality even when wet type toner is employed is proposed in JP-A-3-243973 and JP-A-4-9087.
It appears that tone images are transferred without being affected by irregularities on the surface of receiving material according to these methods. However, since toner images are first transferred onto the primary intermediate transfer medium and then further transferred onto the final receiving material, a lack of toner image, particularly a lack of fine images such as fine lines and fine letters and unevenness in image density are observed in the resulting color images.
Further, toner images remain on the surfaces of light-sensitive element and primary intermediate transfer medium after the transfer process. Therefore, it is necessary to clean the surfaces of light-sensitive element and primary intermediate transfer medium when they are repeatedly employed. This is disadvantageous in that a device for cleaning must be provided and in that the surfaces of light-sensitive element and primary intermediate transfer medium is damaged by cleaning.
As described above, conventional color image forming methods using an intermediate transfer medium have problems in that fully satisfactory color images can not be obtained, in that since the property of intermediate medium is changed, it is difficult to maintain stably its performance over a long period of time, when the intermediate medium is repeatedly used, in that disposable materials must be employed to maintain its performance, and in that a special transfer medium is required.